Chemical imaging of the sulfur-induced deactivation of Cu/ZnO catalyst bodies

The effects of sulfur poisoning on the water–gas shift (WGS) activity of industrial Cu/ZnO/Al2O3 catalyst bodies have been studied. The samples were characterized using chemical imaging methods, including XRD-CT, XAFS mapping, and XRF, in order to understand the process by which accelerated sulfur poisoning leads to catalyst deactivation. After ∼90 h on stream, all catalysts exhibited reduced activity; the higher the H2S concentration, the greater the extent of deactivation. Non-invasive XRD-CT measurements performed on intact samples recovered from the reactor revealed the formation of sulfide phases, including sphalerite (β-ZnS) and crystalline CuS, Cu2S, and CuSO4 phases. These sulfide phases were distributed predominantly as a graduated corona around the sample edge reaching ∼1.5 mm thick for experiments performed in the highest concentration of 500 ppm H2S. XAFS mapping, which is particularly sensitive to the local coordination environment around the element being probed, confirmed the presence of mixed Cu/Zn–O/S coordination environments and that the core of the sample remained sulfur-free. A combination of XRD-CT and XRF revealed that CuS appeared to be mobile under reaction conditions resulting in the redistribution of Cu toward the very edge of the samples. A combination of techniques has therefore demonstrated that H2S deactivation of Cu/ZnO/Al2O3 catalyst bodies occurs via phase transformation of the active Cu/ZnO phase into sulfides and redistribution of these components over the sample instead of Cu active site poisoning by Sads species.